This invention relates to a recovering method of catalytic components of a catalyst, more particularly, it relates to a method of separating and recovering a metal and a carrier from a catalyst which employs, as a carrier, porous silicon carbide prepared from silicon carbide powder using silicon-accumulated biomass such as rice hull and/or rice straw, or a porous silicon carbide prepared from silicon metals, silicon-containing ore, organic silicon compounds, etc. by a chlorinating treatment.
Heretofore, regarding the representative examples of the metal-supported catalysts, there has been known catalysts carried a noble metal such as platinum, rhodium, palladium, etc. Metals which are catalytic components have heretofore been utilized as a reduction-oxidation catalyst for oxidation of ammonia, production of hydrogen cyanide, reforming of petroleum, etc. since these metals absorb hydrogen and oxygen so that the absorbed hydrogen and oxygen are activated.
In recent years, accompanying abrupt popularization of automobiles, air pollution due to their exhaust gases becomes remarkable so that purification of exhaust gas using a catalyst has been carried out by removing unburned hydrocarbon (HC), carbon monoxide (CO), nitrogen oxide (NO.sub.x), etc. which are harmful substances in exhaust gas of automobiles. As a catalyst for purifying exhaust gas of automobiles, platinum group metals including platinum have mainly been used. Also, in combustion equipments such as a boiler, in addition to CO, NO.sub.x, i.e., thermal NO.sub.x is generated by the reaction of nitrogen and oxygen in air at a temperature of 1500.degree. C. or higher whereby pollution is brought about. Thus, in order to restrain the generation of the above thermal NO.sub.x or CO, research on catalytic combustion has been carried out to effect the oxidation reaction at the surface of the catalyst and to conduct perfect combustion at a relatively low temperature which do not generate any thermal NO.sub.x or CO. This catalytic combustion is in principle the same with that of the catalyst for purification of exhaust gas from automobiles.
As the noble metal-supported catalysts, there may be mentioned a pellet catalyst in which a noble metal such as platinum, rhodium, palladium, etc. is supported to an active alumina having a cylindrical shape or spherical shape, or a monolith catalyst in which a catalytic substances such as alumina support and platinum, etc. are coated on a cordierite (2Al.sub.2 O.sub.3.5SiO.sub.2.2MgO) ceramics having numbers of penetrated gas passages, and this catalyst is practically used.
The noble metals such as platinum, etc. used as the active substance for the catalyst are generally expensive since they are rare resources. Thus, it is desired to recover these noble metals from waste catalysts to reuse them, and various recovering methods are being investigated. As the recovering methods, it has been known, for example, (1) the acid dissolution method and (2) the fusing method.
Among these methods, the acid dissolution method of (1) is a method in which noble metals such as platinum, etc. in a waste catalyst is dissolved by aqua regia (hydrochloric acid : nitric acid=3 : 1) to recover them as a chloride after pulverizing a waste catalyst. This method is simple and it has been built up for a long time, but disposal of acids, or reuse or regeneration of a carrier is impossible. Further, for disposal of the recovered material, it must be carried out after removing an acid adhered onto the waste support crushed so that the method has disadvantages that the post-treatment is complicated. Also, among the noble metals, while platinum is dissolved in aqua regia and palladium is dissolved in nitric acid, sulfuric acid or hydrochloric acid, rhodium is insoluble even in aqua regia. Therefore, it is employed the method in which the metal is to be made a chloride by using an active nascent chlorine obtained by electrolyzing hydrochloric acid and then dissolved in hydrochloric acid. However, in this method, there are problems in disposal of waste acids and residue after extraction whereby it has a disadvantage that reuse or regeneration of a carrier is impossible.
On the other hand, the fusing method is a method in which a waste catalyst is heated to a high temperature to fuse a noble metal and the noble metal is recovered by separation from carrier. It is necessary to heat the temperature not less than the melting point of the noble metal (platinum: 1774.degree. C., rhodium: 1966.degree. C., palladium: 1550.degree. C.), and an electric furnace, an arc furnace, a direct current plasma, and the like are used as the heating means. In this method, since a carrier is also heated to a high temperature, the carrier substance causes crystallization and sintering, whereby reuse thereof becomes impossible. That is, for example, activated alumina (.gamma.-alumina) crystallizes into .alpha.-alumina at the temperature of 1000.degree. C. or higher and causes sintering so that the surface area thereof abruptly decreases whereby reuse thereof as a carrier for the catalyst becomes impossible. Further, in spite of the amount of the noble metal supported on the carrier is little (generally about 0.1% by weight), whole catalyst including a carrier (about 99.9% by weight) must be heated, whereby the method has a disadvantage that efficiency in energy is extremely bad. Also, since a part of the noble metal fused remains on a carrier, it has a problem of bad in recovery.
As stated above, there exists many problems in recovery of a noble metal (catalytic substance) from a waste catalyst, and particularly, an effective method in which a carrier substance is effectively recovered and regenerated as well as a noble metal is recovered at the same time has not yet been developed.